1. Field of the Invention
The present invention relates to a surface acoustic wave device used as, for example, a bandpass filter, and more particularly, the present invention relates to a surface acoustic wave device in which an input end and/or an output end includes a pair of balanced signal terminals.
2. Description of the Related Art
In recent years, portable telephones have become smaller and lighter. In order to satisfy these demands, the reduction in the number of component parts, the miniaturization of parts, and the combination of functions must be achieved.
Considering the above circumstances, various surface acoustic wave filters having the function of balanced-to-unbalanced conversion or the function of a balun element, which are used in the RF stage of the portable telephone, have been proposed.
FIG. 30 is a schematic plan view showing the electrode construction of a conventional surface acoustic wave filter having the function of balanced-to-unbalanced conversion.
Here, first to third IDTs 101 to 103 are disposed along the surface acoustic wave propagation direction. Reflectors 104 and 105 are disposed on both sides of the area having the IDTs 101 to 103 provided therein in the surface wave propagation direction. Both the distance between the IDT 101 and the IDT 102 and the distance between the IDT 102 and the IDT 103 are 0.75 λI when a wavelength determined by the pitch of electrode fingers of the IDTs 101 to 103 is λI. The free area between IDTs is made small by thickening the electrode fingers 109 and 110 at both ends of the IDT 102. As a result, the loss caused by the radiation of a bulk acoustic wave is reduced. Moreover, in FIG. 30, terminals 106 and 107 are balanced signal terminals and a terminal 108 is an unbalanced signal terminal.
In the surface acoustic wave filter having the function of balanced-to-unbalanced conversion, the transmission characteristic in each of the passbands between the unbalanced signal terminal 108 and the balanced signal terminal 106 and between the unbalanced signal terminal 108 and the balanced signal terminal 107 is required to be equal in amplitude characteristic and 180 degrees opposite in phase. The condition of being equal in amplitude characteristic is called an amplitude balance and the condition of being 180 degrees different in phase is called a phase balance.
The above-mentioned amplitude balance and phase balance are defined as in the following, when a surface acoustic wave filter having the function of balanced-to-unbalanced conversion is considered to be a three-port device, for example, assuming that the unbalanced input end is port 1 and that the balanced output ends are port 2 and port 3, respectively.Amplitude balance=|A|where A=|20 log S21|−|20 log S31|.Phase balance=|B−180|where B=|∠S21−∠S31|.
Moreover, S21 represents a transfer factor to port 2 from port 1 and S31 represents a transfer factor to port 3 from port 1.
Ideally, it is required that the amplitude balance is 0 dB and the phase balance is 0 dB in the passband of a filter. However, in the construction shown in FIG. 30, when a filter having the function of balanced-to-unbalanced conversion was attempted to be produced, there was a problem in that the balance becomes worse because the number of electrode fingers of the IDT 102 is odd and the number of electrode fingers connected to the balanced signal terminal 106 is greater by one than the number of electrode fingers connected to the balanced signal terminal 107. This problem particularly becomes apparent as the center frequency of the filter increases, and in the filters the center frequency of which is about 1.9 GHz as in the filters for DCS and PCS, adequate balances can not be obtained.